This invention relates to a method of making an improved cathode-ray tube having an internal coating or layer comprising particulate graphite, iron oxide and a silicate binder on the interior walls of the tube, and to the product thereof.
Cathode-ray tubes usually have a conductive internal coating or layer on the interior walls of the tube. This coating is used to carry a high potential of the order of 15 to 35 kilovolts, which is applied at the anode button. A common internal coating consists essentially of particulate graphite, particulate iron oxide, and an alkali silicate binder. Examples of such coatings are disclosed in Table 1 of U.S. Pat. No. 4,041,347 issued Aug. 9, 1977 to S. B. Deal et al. The coating is applied to the walls of the tube by spraying and/or brushing as an aqueous coating composition, then dried, and then baked in air at about 400.degree. to 450.degree. C. for about one hour. Subsequently, the tube is baked and exhausted of gases down to a pressure of about 10.sup.-6 torr, and then tipped-off; that is, hermetically sealed. Subsequently, under some conditions of operation, arcs may be produced in the tube in the vicinity of the electron-gun mount assembly. Associated with these arcs is a surge of current in the internal coating which may be damaging to external circuit components that are connected to the coating through the anode button and/or the stem leads of the tube. It is desirable to limit the amount of arcing current passed by the internal coating, at least to levels that do not damage any of the external circuit components. This desire may be achieved through a modification of the method of making the internal coating according to the novel method.